For example, as disclosed in Japanese Patent-Laid Open Publication No. 62-94443, a vehicular diagnostic system has been known. The vehicular diagnostic system includes a self-diagnostic device which diagnoses a failure in a vehicle. When a failure is detected by the self-diagnostic device, the diagnostic result is transmitted to a center. The center estimates a cause of the failure based on the obtained diagnostic result, and transmits countermeasures for the estimated cause of the failure to the vehicle.
In the above vehicular diagnostic system, when a failure occurs in a vehicle, a self-diagnostic result is transmitted to the center. In recent years, a large number of electronically controlled devices is mounted on a vehicle, and a large volume of data is output from these devices. Therefore, as in the vehicular diagnostic system, if the self-diagnostic result is transmitted to the center, an enormous volume of communication data is transmitted, which may increase communication costs. Also, an enormous volume of communication data increases communication time, which raises a possibility that the center cannot diagnose a failure in the vehicle in real time.
In addition, the number of vehicles communicating with the center has increased, and each vehicle transmits an enormous volume of communication data, which raises a possibility that a communication line is saturated. As a result, a communication disturbance may occur. Particularly, it should be avoided that communication with a vehicle in distress is interrupted. When the center and the vehicle communicate with each other, necessary information should be transmitted/received when necessary.